This invention relates generally to laser gyroscopes and more particularly to pathlength stabilization systems used therein.
As is described in my U.S. Pat. No. 3,741,657, assigned to the same assignee as the present invention, a ring laser having a polarization dispersive path produces four radiant energy waves, each one of such waves having a different frequency, one pair thereof traveling in a clockwise direction and the other pair thereof traveling in a counterclockwise direction. The propagation times of the waves are such that the pair of frequencies of the waves traveling in one direction, say the counterclockwise direction, is positioned between the frequencies of the waves traveling in the opposite, or clockwise, direction. Movement of the laser, for example, by rotation of the system about an axis perpendicular to the path, produces frequency shifts of the pair of waves propagating in one direction through the laser which are opposite to the frequency shifts of the waves moving in the opposite direction through the laser. This, in turn, produces changes in the frequency separation between the lower frequencies of each said pair oppositely to a change in the frequency separation between the higher frequencies of each of said pairs. The difference between such changes is, substantially, a linear function of the rate of said rotation and the relative sense of such difference is indicative of the direction of said rotation.
It is generally desirable in operation of such laser gyroscope that the lower frequency of each pair of frequencies is positioned below the maximum gain frequency. More specifically, the frequencies of each pair are maintained substantially equidistant above and below the maximum gain or center frequency of the laser. As described in my U.S. patent, referenced above, such positioning of the frequencies is accomplished by a pathlength stabilization system wherein a pair of photodetectors is included, one of such photodetectors being responsive to the waves in the lower pair of frequencies and the other one being responsive to the waves in the upper pair of frequencies. The output signals of the pair of photodetectors are filtered and passed to a differential amplifier. The differential amplifier produces a control signal proportional to the difference between the power level of the waves in the upper and lower pairs of frequencies. The control signal is then fed to a piezo-electric crystal included in the support means for one of the reflectors to change the position of such reflector thereby determining the pathlength of the laser. Hence, the pathlength of the laser is stabilized in accordance with the difference in power levels between the upper and lower pairs of frequencies to compensate for mechanical movement of portions of the system with respect to each other, thereby keeping the four frequencies positioned symmetrically about the center maximum gain curve of the laser medium.
While such pathlength stabilization system has been found adequate in many applications, any unbalance between the characteristics of the pair of photodetectors produces concomitant errors in the desired pathlength stabilization. Still further, because both clockwise and counterclockwise waves are used in pathlength stabilization and because such waves pass to the pair of photodetectors through different optical elements, errors in the desired stabilization may result from such elements.